Add mirrored action-cache

Before raising this MR, consider whether the following are required, and complete if so:

• Unit tests
• Metrics
• Documentation update(s)

Description

Add MirroredCache implementation to keep two action-caches in sync.

1. For a write, action-results are written into both underlying caches
2. For a read, the action-result is backfilled if it's missing from one of the caches